Centralizers are used to maintain drilling pipes substantially centred in a well bore during drilling. They are also used to centre casing in the well bore, for example in preparation for cementing.
Centralizers generally comprise a plurality of outwardly curved spring bows which extend between a top and a bottom support ring. The outwardly curved spring bows offer considerable resistance to the introduction of a pipe provided with the centralizer into a well bore or an enclosing casing pipe. However, whilst the centring force of the centralizer must be high to operate effectively existing standards require that the insertion forces do not exceed a predetermined limit. These demands can be met when there is sufficient space between the external periphery of the pipe to be inserted and the well bore (or enclosing casing pipe). However, if the annular gap is small, for example, only to a few millimetres, it has hitherto been impossible to use satisfactory centring devices.
An attempt was made to obviate these difficulties as disclosed in German Utility Model 89 03 038. In particular, in one embodiment the centralizer can be pressed against the periphery of the pipe due to a clearance for the support rings. However, the centralizer has to be assembled around the pipe. In another embodiment stop rings are welded to the external periphery of the pipe. However, this is technically undesirable and time-consuming.